In Qin Shihuang's idea, China is a country surrounded by oceans, but after Qin Shihuang unified the six countries, he even occupied the East China Sea and the West Sea. Qin Shihuang also wanted to occupy the South China Sea and the North Sea in order to achieve real reunification. At that time, Beihai was occupied by Xiongnu, and then he sent 300,000 troops to attack Xiongnu. During the attack on Huns, Qin Shihuang also ordered the construction of the Great Wall.
When the Great Wall was completed, the army at that time could attack the Huns based on the Great Wall and finally achieve the goal of occupying the North Sea. But this process is quite long. It took a long time to build the Great Wall, let alone attack the Huns.
Compared with the North Sea, the South China Sea is more vulnerable to attack. If you want to occupy the South China Sea now, you must first occupy Baiyue, which is a wild land, and Qin Shihuang thought it was easy to win. So when he attacked Baiyue, he sent 500,000 troops.
The second reason why Qin Shihuang wanted to attack Xiabaiyue was to eliminate hidden dangers in the south. At first Baiyue was at war with Chu, but Chu did not completely win Baiyue. Because the environment in Baiyue is very bad, there are many swamps, and there are many nationalities here. When the state of Chu defeated Baiyue, some ethnic groups in Baiyue would resist after a while. So if Qin Shihuang takes Baiyue, he will kill two birds with one stone.